"The streets are almost completely empty and there is an unannounced curfew," said a resident, speaking from the town of Rastan which was hit in the first day of Russian air strikes.
Islamic State also canceled prayers in areas it controls, according to activists from its de facto capital Raqqa.
A Russian air strike on Thursday destroyed a mosque in the town of Jisr al-Shughour, captured from government forces by an alliance of Islamist insurgents earlier this year, activists said.
Western countries and Russia say they have a common enemy in Islamic State.
But they also have very different allies and opposing views of how to resolve a war that has killed at least 250,000 people and driven more than 10 million from their homes.
Washington and its allies oppose both Islamic State and Assad, blaming him for attacks on civilians that have radicalized the opposition and insisting that he has no place in a post-war settlement.
Lebanese sources have told Reuters that hundreds of Iranian troops have also arrived in recent days in Syria to participate in a major ground offensive alongside government troops and their Lebanese and Iraqi Shi'ite militia allies.
Syria's foreign minister said the U.S.-led coalition's campaign against Islamic State was bound to fail.
"Air strikes are useless unless they are conducted in cooperation with the Syrian army, the only force in Syria that is combating terrorism," Walid al-Moualem said in a speech to United Nations General Assembly in New York.
Moscow said on Friday its latest strikes had hit 12 Islamic State targets, but most of the areas it described were in western and northern parts of the country, while Islamic State is mostly present in the east.
The Russian Defence Ministry said its Sukhoi-34, Sukhoi-24M and Sukhoi-25 warplanes had flown 18 sorties hitting targets that included a command post and a communications center in the province of Aleppo, a militant field camp in Idlib and a command post in Hama.
A Defence Ministry official, Igor Konashenkov, later told Russian news agencies the Air Force had conducted 14 flights in Syria on Friday and made six strikes against Islamic State targets. The ministry was not immediately available to comment on the apparent discrepancy with the earlier figures.